The purpose of a catalytic converter is to convert pollutant materials in engine or turbine exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxides, etc., to carbon dioxide, nitrogen and water. Conventional catalytic converters utilize an oval cross-section ceramic honeycomb monolith 4 to 8 inches long having square, circular, triangular, or hexagonal axially extending straight through openings or cells with a noble metal catalyst deposited in the cells. Other types of catalytic converters include catalyst coated refractory metal oxide beads or pellets, e.g., alumina beads, and a corrugated thin metal foil monolith, e.g., ferritic stainless steel foil monolith, having a catalyst supported on the surface, usually a refractory metal oxide surface-e. The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such metals. The catalyst catalyzes a chemical reaction, mainly oxidation, whereby pollutant materials in the exhaust are converted to harmless by-products which then pass through the exhaust system to the atmosphere.
However, conversion is not efficient initially when the exhaust gases and the converter are relatively cold. To be effective at a high conversion rate, the catalyst and the surface of the converter must be at a minimum temperature, e.g., 390.degree. F. for carbon monoxide, 570.degree. F. for volatile organic compounds (VOC) including unburned hydrocarbons, and 1000.degree. F. for methane or natural gas. Otherwise, conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. Once the exhaust system has come to its operating temperature, the catalytic converter is optimally effective. Hence, it is necessary to contact relatively cold exhaust gases with hot catalyst to effect satisfactory conversion at engine start-up. Both compression ignited and spark ignited internal combustion engines have this need. Gas turbines also have this need.
To achieve initial heating of the catalyst prior to or upon engine start-up, current practice provides an electrically heatable catalytic converter formed usually of a corrugated thin metal foil monolith which is connected to a voltage source, e.g., a 12 volt to 108 volt automotive battery, and power supplied before, or at the time of, and during and after engine ignition to elevate and maintain the temperature of the catalyst to at least 650.degree. F. plus or minus 20.degree. F. The initial heat up time prior to engine ignition is from 2 to 30 seconds, and post crank heating is generally on demand. In some cases, heating is continuous from ignition to shut-down.
The resistance of the corrugated thin metal monolith is used to heat the converter and accordingly electric power must be supplied to the monolith at opposite ends of a corrugated thin metal strip or strips from which the monolith is made. To accomplish this, at least one electrode must extend through the housing and be electrically isolated from the housing. Where only one electrode is used, the housing, being attached to the chassis, becomes the opposite pole of the voltage source. Where two electrodes of opposite charge are used, both must extend, in electrically- isolated manner, through the housing and be attached to the monolith.
With prior insulated terminals or electrodes, gas leakage has occurred and it has now been found desirable to pneumatically seal the electrode or electrodes. Leakage is unacceptable because it causes oxygen sensors in the vehicle's emission system to malfunction.
Reference may be had to U.S. Pat. No. 4,711,009 to Cornelison et al dated Dec. 8, 1987 for details of a process for corrugating and coating thin metal foil strips and applying the catalyst, which process, with or without the final steps of creasing and folding the strip, may be used herein.
In one embodiment, lengths of corrugated strip are secured as by welding to a tubular central core member, closed at at least one end, and spirally wound about the core. The outer ends are brazed to an outer metallic shell.
In the following description, reference will be had to "ferritic" stainless steel. A suitable formulation for this alloy will be found in U.S. Pat. No. 4,414,023 dated Nov. 8, 1983 to Aggen. A specific ferritic stainless steel alloy useful herein contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, praseodymium, and mixtures of two or more thereof, balance iron, and trace steel making impurities.
Another useful alloy is Haynes 214 described in U.S. Pat. No. 4,671,931 dated Jun. 9, 1987 to Herchenroeder and is an alloy of nickel/chromium/aluminum/iron. A specific example contains 75% nickel, 16% chromium, 4.5% aluminum, 3% iron, trace amounts of one or more rare earth metals, 0.05% carbon, and steel making impurities.
Ferritic stainless steel and Haynes 214 are examples of high temperature resistive, corrosion resistant metal alloys useful in making the electrically heatable catalytic converters hereof. Suitable alloys must be able to withstand temperatures of 900.degree. C. to 1100.degree. C. over prolonged periods.
In the following description, reference will also be made to fibrous ceramic mat or insulation. Reference may be had to U.S. Pat. No. 3,795,524 dated Mar. 5, 1974 to Sowman for formulation and manufacture of ceramic fibers and mats useful herein. See also the U.S. Pat. No. 3,916,057 to Hatch dated Oct. 28, 1975. One such ceramic fiber material is currently available from 3-M under the Registered Trademark "NEXTEL" 312 Woven Tape and is especially useful herein. Ceramic fiber mat is available under the Registered Trademark "INTERAM" also from 3-M.
A brazing foil, which is an alloy of nickel, chromium, silicon and boron useful herein is available commercially from Allied Metglas Products of Parsippany, N.J.